The disclosure relates to vapor compression systems. More particularly, the disclosure relates to surge control of multi-stage centrifugal compressors in vapor compression systems.
One example of a vapor compression system involves a chiller. The exemplary chiller involves a two-stage centrifugal compressor driven by an electric motor. The main refrigerant flowpath through the exemplary system passes sequentially from an outlet of the compressor through a condenser, an economizer (e.g., a flash tank economizer), an expansion device, and a cooler, returning from the cooler to the compressor inlet. An economizer line may extend from the economizer to an interstage of the compressor.
The exemplary prior art chiller uses a hot gas bypass to provide stable operation at low loads. The exemplary bypass is from discharge conditions (e.g., between the compressor outlet and condenser inlet) to cooler inlet conditions (e.g., downstream of the expansion device). Flow along the bypass flowpath is governed by a hot gas bypass valve, in turn controlled by a system controller. When compressor load falls below a set level, the exemplary controller opens the bypass valve. This causes an increase of load to the compressor resulting in stable operation.